Gasoline, the fuel for many automotive vehicles, is a volatile liquid subject to potentially rapid evaporation, in response to diurnal variations in the ambient temperature. Thus, the fuel contained in automobile gas tanks presents a major source of potential emission of hydrocarbons into the atmosphere. Such emissions from vehicles are termed ‘evaporative emissions’, and those vapors can be emitted vapors even when the engine is not running.
In response to this problem, industry has incorporated evaporative emission control systems (EVAP) into automobiles, to prevent fuel vapor from being discharged into the atmosphere. EVAP systems include a canister (the carbon canister) containing adsorbent carbon) that traps fuel vapor. Periodically, a purge cycle feeds the captured vapor to the intake manifold for combustion, thus reducing evaporative emissions.
Hybrid electric vehicles, including plug-in hybrid electric vehicles (HEV's or PHEV's), pose a particular problem for effectively controlling evaporative emissions. Although hybrid vehicles have been proposed and introduced in a number of forms, these designs all provide a combustion engine as backup to an electric motor. Primary power is provided by the electric motor, and careful attention to charging cycles can produce an operating profile in which the engine is only run for short periods. Systems in which the engine is only operated once or twice every few weeks are not uncommon. Purging the carbon canister can only occur when the engine is running, of course, and if the canister is not purged, the carbon pellets can become saturated, after which hydrocarbons will escape to the atmosphere, causing pollution.
EVAP systems are generally sealed to prevent the escape of any hydrocarbons. These systems require periodic leak detection tests to identify potential problems.
A requirement for leak testing, of course, is a standard against which to measure. In general, leak standards are expressed in terms of the maximum allowable orifice size. This field is relatively new, however, and regulatory bodies often change standards, requiring adaptation of standard leak detection procedures. Such changes often necessitate modifications in the measuring equipment, which imposes higher costs. One aspect under considerable discussion is the maximum allowable orifice size—that is, size limit for the largest allowable leak. Commonly used test equipment provides a reference orifice to establish a reference pressure level and thus any change in the maximum allowable orifice size would require changes in the reference orifice as well.
Understandably, changes to test systems, such as modifications to the orifice size, exact a toll on the manufacturers and service providers. This leaves alternatives to accommodate multiple orifices or attempts to determine the reference pressure efficiently during EVAP leak tests substantially unchallenged.